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Glennys R. Farrar

Publications and source records attributed to Glennys R. Farrar.

At least 73 records · Page 4Linked to original sources
arXiv2014

Magnetic Deflections of Ultra-High Energy Cosmic Rays from Centaurus A

We present the results of a study that simulates trajectories of ultra-high energy cosmic rays from Centaurus A to Earth, for particle rigidities from $E/Z = 2$ EV to 100 EV, i.e., covering the possibility of primary particles as heavy as Fe nuclei with energies exceeding 50 EeV. The Galactic magnetic field is modeled using the recent work of Jansson and Farrar (JF12) which fitted its parameters to match extragalactic Faraday rotation measures and WMAP7 synchrotron emission maps. We include the random component of the GMF using the JF12 3D model for $B_{\rm rand}(\vec{r})$ and explore the impact of different random realizations, coherence length and other features on cosmic ray deflections. Gross aspects of the arrival direction distribution such as mean deflection and the RMS dispersion depend mainly on rigidity and differ relatively little from one realization to another. However different realizations exhibit non-trivial substructure whose specific features vary considerably from one realization to another, especially for lower rigidities. At the lowest rigidity of 2 EV, the distribution is broad enough that it might be compatible with a scenario in which Cen A is the principle source of all UHECRs. No attempt is made here to formulate a robust test of this possibility, although some challenges to such a scenario are noted.

astro-ph.HE↗
arXiv2014

The Galactic Magnetic Field and Ultrahigh-Energy Cosmic Ray Deflections

Our understanding of the Galactic magnetic field (GMF) has increased considerably in recent years, while at the same time remaining far from adequate. By way of illustration, the Jansson and Farrar (2012) (JF12) GMF model is described, emphasizing how it is constrained and which features are robust or likely to change, as modelling and constraining data improve. The most urgent requirements for the next phase of modelling are a more realistic model for the relativistic electron distribution (in order to reduce the systematic error associated with interpreting synchrotron data) and a better theoretical understanding of the origin of the large-scale coherent field (in order to develop a better phenomenological parameterization of the field). Even in its current stage of development, the JF12 model allows some important conclusions about UHECR deflections in the GMF to be formulated.

astro-ph.HE↗
arXiv2013

Tentative identification of the source of four UHECRs, and implications thereof

Four UHECRs in the combined HiRes and AGASA datasets are backtracked in the Galactic magnetic field. They point to a common source which is localized to within 1 degree, if they are protons as is shown to be the most probable charge assignment. A Swift-BAT hard X-ray AGN in the galaxy CGCG 291-028 is the only notable source candidate within the source locus and within the GZK distance horizon. The spectrum of the four events is consistent with production in a transient event such as a stellar tidal disruption flare. Under the assumption the UHECRs were produced in CGCG 291-028, the total energy of UHECRs produced by the transient can be estimated and extragalactic magnetic deflections can be constrained. If CGCG 291-028 is indeed the source of the UHECRs, observations of its present state should elucidate the phenomenon of UHECR acceleration.

astro-ph.HE↗
arXiv2013

A new physical phenomenon in ultra-high energy collisions

We show that combining the published Pierre Auger Observatory measurements of the longitudinal_and_ lateral properties of UHE atmospheric showers, points to an unforeseen change in the nature of particle interactions at ultrahigh energy. A "toy model" of UHE proton-air interactions is presented which provides the first fully consistent description of air shower observations. It demonstrates that the observed energy dependence of the depth-of-shower-maximum distribution may not indicate a transition to a heavier composition, as commonly assumed. While fundamentally phenomenological, the model is based on considerations of how the normal vacuum of QCD might be vaporized and chiral symmetry restored by the extreme energy densities produced in UHE collisions. Whatever its origin, understanding this unexpected phenomenon opens exciting directions in particle physics and may impact Early Universe cosmology.

hep-ph↗
arXiv2012

Galactic magnetic deflections and Centaurus A as a UHECR source

We evaluate the validity of leading models of the Galactic magnetic field for predicting UHECR deflections from Cen A. The Jansson-Farrar 2012 GMF model (JF12), which includes striated and random components as well as an out-of-plane contribution to the regular field not considered in other models, gives by far the best fit globally to all-sky data including the WMAP7 22 GHz synchrotron emission maps for Q, U, and I and ~40,000$ extragalactic Rotation Measures (RMs). Here we test the models specifically in the Cen A region, using 160 well-measured RMs and the Polarized Intensity from WMAP, nearby but outside the Cen A radio lobes. The JF12 model predictions are in excellent agreement with the observations, justifying confidence in its predictions for deflections of UHECRs from Cen A. We find that up to six of the 69 Auger events above 55 EeV are consistent with originating in Cen A and being deflected <18 degrees; in this case three are protons and three have Z=2-4. Others of the 13 events within 18 degrees must have another origin. In order for a random extragalactic magnetic field between Cen A and the Milky Way to appreciably alter these conclusions, its strength would have to be > ~ 80 nG -- far larger than normally imagined.

astro-ph.HE↗
arXiv2012

The Galactic Magnetic Field

With this Letter, we complete our model of the Galactic magnetic field (GMF), by using the WMAP7 22 GHz total synchrotron intensity map and our earlier results to obtain a 13-parameter model of the Galactic random field, and to determine the strength of the striated random field. In combination with our 22-parameter description of the regular GMF, we obtain a very good fit to more than forty thousand extragalactic Faraday Rotation Measures (RMs) and the WMAP7 22 GHz polarized and total intensity synchrotron emission maps. The data calls for a striated component to the random field whose orientation is aligned with the regular field, having zero mean and rms strength ~20% larger than the regular field. A noteworthy feature of the new model is that the regular field has a significant out-of-plane component, which had not been considered earlier. The new GMF model gives a much better description of the totality of data than previous models in the literature.

astro-ph.GA↗
arXiv2012

Tidal Disruption Flares as the Source of Ultra-high Energy Cosmic Rays

The optical spectral energy distributions of two tidal disruption flares identified by van Velzen et al. (2011) in archival SDSS data, are found to be well-fit by a thin-accretion-disk model. Furthermore, the inferred Supermassive Black Hole mass values agree well with the SMBH masses estimated from the host galaxy properties. Integrating the model SEDs to include shorter wavelength contributions provides an estimate of the bolometric luminosities of the accretion disks. The resultant bolometric luminosities are well in excess of the minimum required for accelerating UHECR protons. In combination with the recent observational estimate of the TDF rate (van Velzen and Farrar, these Proceedings), the results presented here strengthen the case that transient jets formed in tidal disruption events may be responsible for accelerating all or most UHECRs.

astro-ph.HE↗
arXiv2012

The rate of stellar tidal disruption flares from SDSS data

We report an observational estimate of the rate of stellar tidal disruption flares (TDFs), based on our (successful) search for these events in archival Sloan Digital Sky Survey (SDSS) multi-epoch imaging data. Our pipeline took advantage of the excellent astrometry of SDSS to separate nuclear flares from supernovae. The 10 year baseline and the high cadence of the observations facilitate a clear-cut identification of variable active galactic nuclei. We found 186 nuclear flares, of which two are excellent TDF candidates. To compute the rate of (optical) stellar tidal disruption events, we simulate our entire pipeline to obtain the efficiency of detection for a given light curve. Using a simple model to extrapolate the observed light curves forward and backward in time, we find our best-estimate of the TDF rate: 3x10-5 per galaxy per year. In addition, we give a model-independent upper limit to the TDF rate: < 3x10-4 per galaxy per year (90% CL).

astro-ph.CO↗
arXiv2012

Chandra observations and classification of AGN-candidates correlated with Auger UHECRs

We report on Chandra X-ray observations of possible-AGNs which have been correlated with Ultra-high Energy Cosmic Rays (UHECRs) observed by the Pierre Auger Collaboration. Combining our X-ray observations with optical observations, we conclude that one-third of the 21 Veron-Cetty Veron (VCV) galaxies correlating with UHECRs in the first Auger data-release are actually not AGNs. We review existing optical observations of the 20 VCV galaxies correlating with UHECRs in the second Auger data-release and determine that three of them are not AGNs and two are uncertain. Overall, of the 57 published UHECRs with |b|>10 degrees, 22 or 23 correlate with true AGNs using the Auger correlation parameters. We also measured the X-ray luminosity of ESO139-G12 to complete the determination of the bolometric luminosities of AGNs correlating with UHECRs in the first data-set. Apart from two candidate sources which require further observation, we determined bolometric luminosities for the candidate galaxies of the second dataset. We find that only two of the total of 69 published UHECRs correlate with AGNs (IC5135 and IC4329a) which are powerful enough in their steady-state to accelerate protons to the observed energies of their correlated UHECRs. The GZK expectation is that about 45% of the sources of UHECRs above 60 EeV should be contained within the z<0.018 volume defined by the Auger scan analysis, so an observed level of 30-50% correlation with weak AGNs is compatible with the suggestion that AGNs experience transient high-luminosity states during which they accelerate UHECRs.

astro-ph.HE↗
arXiv2012

A New Model of the Galactic Magnetic Field

A new, much improved model of the Galactic Magnetic Field (GMF) is presented. We use the WMAP7 Galactic Synchrotron Emission map and more than forty thousand extragalactic rotation measures to constrain the parameters of the GMF model, which is substantially generalized compared to earlier work to now include an out-of-plane component (as suggested by observations of external galaxies) and striated-random fields (motivated by theoretical considerations). The new model provides a greatly improved fit to observations. Consistent with our earlier analyses, the best-fit model has a disk field and an extended halo field. Our new analysis reveals the presence of a large, out-of-plane component of the GMF; as a result, the polarized synchrotron emission of our Galaxy seen by an edge-on observer is predicted to look intriguingly similar to what has been observed in external edge-on galaxies. We find evidence that the cosmic ray electron density is significantly larger than given by GALPROP, or else that there is a widespread striated component to the GMF.

astro-ph.GA↗
arXiv2011

Optical discovery of probable stellar tidal disruption flares

Using archival SDSS multi-epoch imaging data (Stripe 82), we have searched for the tidal disruption of stars by super-massive black holes in non-active galaxies. Two candidate tidal disruption events (TDEs) are identified. They have optical black-body temperatures 2 10^4 K and observed peak luminosities M_g=-18.3 and -20.4; their cooling rates are very low, qualitatively consistent with expectations for tidal disruption flares. Their properties are examined using i) SDSS imaging to compare them to other flares observed in the search, ii) UV emission measured by GALEX and iii) spectra of the hosts and of one of the flares. Our pipeline excludes optically identifiable AGN hosts, and our variability monitoring over 9 years provides strong evidence that these are not flares in hidden AGNs. The spectra and color evolution of the flares are unlike any SN observed to date, their strong late-time UV emission is particularly distinctive, and they are nuclear at high resolution, arguing against their being first cases of a previously-unobserved class of SNe or more extreme examples of known SN types. Taken together, the observed properties are difficult to reconcile with a SN or AGN-flare explanation, although an entirely new process specific to the inner few-hundred parsecs of non-active galaxies cannot be excluded. Our observed rate and method show the feasibility of obtaining a candidate TDE sample of hundreds of events and O(1) purity, using geometric resolution and host and flare color alone. A by-product of this work is quantification of the power-spectrum of extreme flares in AGNs.

astro-ph.CO↗
arXiv2011

Chandra observations of AGN-candidates correlated with Auger UHECRs

The Auger observatory has observed a possible correlation between Ultrahigh Energy Cosmic Rays (UHECRs) above 57 EeV and nearby candidate Active Galactic Nuclei (AGN) from the Veron-Cetty Veron catalog (VCV). In this paper we report on Chandra X-ray observations of 10 unconfirmed VCV AGN-candidates and luminous IR galaxies correlating with the first set of Auger UHECRs, to determine whether or not they have active nuclei. The X-ray data, when combined with optical luminosities, show that in fact none of the 10 galaxies have a significant AGN component; if there is any nuclear activity at all, it is weak rather than obscured. This reduces the number of UHECRs in the original Auger dataset possibly correlating with AGNs from 20 of 27 down to 14 of 27. We also used Chandra to measure the X-ray luminosity of ESO 139-G12, an AGN which correlates with 2 of the Auger UHECRs, to obtain the first estimate of its bolometric luminosity; this completes the determination of the bolometric luminosities of all correlating AGNs. Taking our results into account, only one of the 27 UHECRs in the original Auger data-release is correlated on a few-degree angular scale with an identified AGN that is powerful enough in its steady-state to accelerate protons to the observed energies, according to conventional acceleration mechanisms. Intriguingly, approximately 60% of the UHECRs with $|b|>10^\circ$ do correlate with genuine AGNs, but these are too weak to meet the acceleration criterion for protons; this may be an indication that AGNs experience transient high-luminosity states which can accelerate UHECRs. To determine the source(s) and composition of UHECRs through statistical correlation studies requires reliable, complete and uniform catalogs of identified AGNs; our a posteriori inspection of ambiguous source candidates underscores the inadequacies of the VCV catalog in this respect.

astro-ph.CO↗
arXiv2010

Correlations between Ultrahigh Energy Cosmic Rays and Infrared-Luminous Galaxies

We confirm the UHECR horizon established by the Pierre Auger Observatory using the heterogeneous Veron-Cetty Veron (VCV) catalog of AGNs, by performing a redshift-angle-IR luminosity scan using PSCz galaxies having infrared luminosity greater than 10^{10}L_sun. The strongest correlation -- for z < 0.016, psi = 2.1 deg, and L_ir > 10^{10.5}L_sun -- arises in fewer than 0.3% of scans with isotropic source directions. When we apply a penalty for using the UHECR energy threshold that was tuned to maximize the correlation with VCV, the significance degrades to 1.1%. Since the PSCz catalog is complete and volume-limited for these parameters, this suggests that the UHECR horizon discovered by the Pierre Auger Observatory is not an artifact of the incompleteness and other idiosyncrasies of the VCV catalog. The strength of the correlation between UHECRs and the nearby highest-IR-luminosity PSCz galaxies is stronger than in about 90% percent of trials with scrambled luminosity assignments for the PSCz galaxies. If confirmed by future data, this result would indicate that the sources of UHECRs are more strongly associated with luminous IR galaxies than with ordinary, lower IR luminosity galaxies.

astro-ph.HE↗
arXiv2009

Constraining models of the large scale Galactic magnetic field with WMAP5 polarization data and extragalactic Rotation Measure sources

We introduce a method to quantify the quality-of-fit between data and observables depending on the large scale Galactic magnetic field. We combine WMAP5 polarized synchrotron data and Rotation Measures of extragalactic sources in a joint analysis to obtain best fit parameters and confidence levels for GMF models common in the literature. None of the existing models provide a good fit in both the disk and halo regions, and in many instances best-fit parameters are quite different than the original values. We note that probing a very large parameter space is necessary to avoid false likelihood maxima. The thermal and relativistic electron densities are critical for determining the GMF from the observables but they are not well constrained. We show that some characteristics of the electron densities can already be constrained using our method and with future data it may be possible to carry out a self-consistent analysis in which models of the GMF and electron densities are simultaneously optimized.

astro-ph.GA↗
arXiv2009

Correlations between Ultrahigh Energy Cosmic Rays and AGNs

We investigate several aspects of the correlations reported by the Pierre Auger Observatory between the highest energy cosmic rays (UHECRs) and galaxies in the Veron-Cetty Veron (VCV) catalog of AGNs. First, we quantify the extent of the inhomogeneity and impurity of the VCV catalog. Second, we determine how the correlation between the highest energy Auger UHECRs and VCV galaxies is modified when only optically-identified AGNs are used. Finally, we measure the correlation between the published Auger UHECRs and the distribution of matter. Our most important finding is that the correlation between UHECRs and AGNs is too strong to be explained purely by AGNs tracing the large scale distribution of matter, indicating that (barring the correlation being a statistical fluke) some substantial fraction of UHECRs are produced by AGNs. We also find that once we take into account the heavy oversampling of the VCV catalog in the Virgo region, the lack of UHECR events from that region is not incompatible with UHECR having AGN sources.

astro-ph.HE↗
arXiv2008

Generalized Maximum Likelihood Method for Ultrahigh Energy Cosmic Rays

The Maximum Likelihood Method is generalized to include effects important for UHECR applications. The new approach can incorporate source distance constraints implied by the observed CR energy and can allow for energy uncertainties, possible deflection in magnetic fields, multiple source types, and a spectrum of CR composition. It can be efficiently implemented and does not require the unphysical "isotropic" assumption for unidentified sources. The approach optimizes the utility of UHECR data to discriminate between source classes and can help constrain galactic and extragalactic magnetic fields. Aspects of the method are directly applicable in other contexts, such as TeV gamma ray astrophysics.

astro-ph↗
arXiv2008

Giant AGN Flares and Cosmic Ray Bursts

We predict a new class of very intense, short-duration AGN flares capable of accelerating the highest energy cosmic rays, resulting from the tidal disruption of a star or from a disk instability. The rate and power of these flares readily explains the observed flux and density statistics of UHECRs. The photon bursts produced by the predicted AGN flares are discussed; they may soon be detectable. Observations are shown to exclude that continuous jets of powerful Active Galactic Nuclei are the sole source of ultrahigh energy cosmic rays; the stringent requirements for Gamma Ray Bursts to be the source are delineated.

astro-ph↗